Ayaka-chan wa Hiroko Senpai ni Koishiteru

Soft and bubbly office lady Ayaka is madly in love with her senior at work, Hiroko! Two lovestruck coworkers who both think the other is straight totally crush on each other...
(Source: MangaPlaza)
Note: Previously serialized under the title "Ayaka-chan wa Hiroko Senpai wo Otoshitai" (彩香ちゃんは弘子先輩を落としたい)